Re: [Qemu-devel] [PATCH v4 17/28] qapi: Factor out JSON number formatting

[Eric Blake]

On 06/02/2016 09:02 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
> 
>> Pull out a new qstring_append_json_number() helper, so that all
>> JSON output producers can use a consistent style for printing
>> floating point without duplicating code (since we are doing more
>> data massaging than a simple printf format can handle).  (For
>> now, there is only one client, but later patches will use it.)
>>
>> Adding a return value will let callers that care diagnose the
>> situation where an attempt was made to output invalid JSON (which
>> does not specify Infinity or NaN). None of the current callers
>> care, but a future patch wants to make it possible to turn this
>> situation into an error.

> 
> Suggest:
> 
>  * Return 0 if the number is finite, as required by RFC 7159, else -1.
> 
> The return value makes some sense only for symmetry with
> qstring_append_json_string().  Without that, I'd ask you to keep this
> function simple.  Callers could just as easily test isfinite()
> themselves.

I'm actually thinking of modifying this, given the recent thread
pointing out that libvirt chokes hard on JSON extensions:

https://lists.gnu.org/archive/html/qemu-devel/2016-05/msg04398.html

That is, for symmetry with qstring_append_json_string(), I'm thinking of
changing NaN to 0 and Inf to DBL_MAX, and always outputting a finite
value, in addition to returning -1 to inform the caller that a
substitution was made, so that the output is always strict JSON.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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